Reduction of alcohols to the corresponding hydrocarbon is usually accomplished in two consecutive steps. First, the hydroxyl is reacted to generate a sulfonate, halide or epoxide, and then these derivatives are reacted with a reducing agent (S. Hanessian, 1996). Alternative strategies include Barton-MacCombie radical reduction of a sulfur manipulated hydroxyl group (S. Z. Zard, 1997). By oxidizing the alcohol to a ketone the Wolf-Kischner or Clemmensen reductions generate the corresponding hydrocarbons (D. Todd, 1948, E. L. Martin, 1942). All these methodologies suffer from being two step processes and the fact that stoichiometric amount of reagents are used.
The formal catalytic reduction of alcohols to their corresponding alkanes is a rare transformation in organic chemistry. Most studies have used transition metals based on palladium (H. van Bekkum, 1971, 2007), but also ruthenium (M. Schlaf, 2009), and rhodium (R. Prins, 2000) have been reported. Traditionally, hydrogen gas has been employed in catalytic hydrogenolysis to generate the alkane and water as side product.
The formal reduction of alcohols to the corresponding hydrocarbon can also be accomplished by elimination-reduction methodology (G. W. Huber, 2004, J. A. Dumesic, 2006, 2007, 2008, 2009) at high reaction temperatures and pressure.
More recently, formic acid has been employed as the source of hydrogen and the reaction is termed catalytic transfer hydrogenolysis (H. Chen, 2009, G. Lu, 2006). The use of formic acid as hydrogen source has many advantages in regards to handling, transport, and storage and can easily be generated from hydrogen gas and carbon dioxide (P. G. Jessop, 2004). Furthermore, formic acid is not explosive and is not hazardous compared to methanol. Because the generated carbon dioxide can be recycled into formic acid with the addition of hydrogen gas, the reaction is atom efficient where only water is formed as side-product in the net reaction. A problem with the reported procedures in which formic acid has been used as the reductant in the transfer hydrogenolysis of alcohols, is a competing disproportionation reaction that limits the efficiency of the process. In fact we have found that the reported transfer hydrogenolysis reported actually is a tandem disproportionation and transfer hydrogenation process in which the formed ketone from the disproportionation is continuously reduced by transfer hydrogenation to regenerate the alcohol. That is, the alcohol is favored as hydrogen donor over the formic acid. That is, the reaction to generate 50% hydrocarbon is fast, followed by a slower process where the formic acid reduces the ketone to the alcohol in a transfer hydrogenation followed by a transfer hydrogenolysis (FIG. 1).